The Rock Cycle and Involved Processes

The Rock Cycle, Source: Steven, E. (2015)

Definition of Igneous Rocks

• Igneous Rocks are formed from the solidification of molten magma or lava.
  • Magma: Molten rock beneath the Earth’s surface.
  • Lava: Molten rock that has reached the surface.

Source: Physical Geology, First University of Saskatchewan Edition

Magma and Magma Composition

• Magma is a molten rock, composed of:
  • Silica (SiO₂): The most important component, affects viscosity and type of igneous rock.
  • Other elements: Aluminum, iron, magnesium, calcium, potassium, sodium, and water (H₂O).
• Magma Types: Felsic, Intermediate, Mafic, and Ultramafic (based on silica content).

Factors Affecting Magma

• Viscosity: Resistance to flow, controlled by silica content and temperature.
  • Higher silica = higher viscosity (e.g., felsic magmas).
  • Lower silica = lower viscosity (e.g., mafic magmas).
• Fluids Content: Water and gases lower viscosity, making magma more explosive.
• Temperature: Higher temperature = lower viscosity; mafic magmas are hotter than felsic magmas.

Bowen’s Reaction Series

• Describes the sequence of mineral crystallization from magma.
  • Discontinuous Series: Olivine → Pyroxene → Amphibole → Biotite.
  • Continuous Series: Plagioclase Feldspar (Calcium-rich → Sodium-rich).
  • End Products: K-Feldspar, Muscovite, Quartz.

Bowen’s reaction series, Source: https://geologyhub.com

Classification of Igneous Rocks

• Place of Formation
• Texture
• Colour
• Silica Content

Classification by Place of Formation

• Intrusive Igneous Rocks (Plutonic): Form beneath the Earth’s surface, slow cooling, large crystals (e.g., Granite, Diorite).
• Extrusive Igneous Rocks (Volcanic): Form on the Earth’s surface, rapid cooling, small crystals (e.g., Basalt, Rhyolite).

Source: Physical Geology, First University of Saskatchewan Edition

Classification by Texture

• Aphanitic Texture: Fine-grained, rapid cooling (e.g., Basalt, Andesite).
• Phaneritic Texture: Coarse-grained, slow cooling (e.g., Granite, Gabbro).
• Porphyritic texture: A phenocryst (large crystal) enclosed in a “groundmass” of smaller crystals.

Source: Physical Geology, First University of Saskatchewan Edition

Volcanic Rocks Without Phenocrysts

• Vesicular Texture: Formed by gas bubbles trapped in magma, creating small cavities in volcanic rocks like basalt and pumice.
• Amygdaloidal Texture: Occurs when vesicles in volcanic rocks are filled with secondary minerals, forming rounded or almond-shaped inclusions.

Source: Physical Geology, First University of Saskatchewan Edition

Glassy Volcanic Rocks

• Crystal size is a function of cooling rate.
• The faster magma or lava cools, the smaller the crystals it contains.
• It is possible for lava to cool so rapidly that no crystals can form.
• The result is called volcanic glass.
• Volcanic glass can be smooth like obsidian or vesicular like scoria (mafic) and pumice.
• Pumice can float on water because of its low-density felsic composition and enclosed vesicles.

Source: Physical Geology, First University of Saskatchewan Edition

Classification by Colour

• Melanocratic: Dark-colored rocks rich in mafic minerals (e.g., Basalt, Gabbro).
• Leucocratic: Light-colored rocks rich in felsic minerals (e.g., Granite, Rhyolite).
• Mesocratic: Intermediate color, balanced mafic and felsic minerals (e.g., Diorite, Andesite).

Classification by Silica Content or (Proportion of Dark Minerals)

• Ultramafic Rocks: Very low silica, high in iron and magnesium (e.g., Peridotite).
• Mafic Rocks: Low silica, rich in iron and magnesium (e.g., Basalt, Gabbro).
• Intermediate Rocks: Moderate silica content (e.g., Diorite, Andesite).
• Felsic Rocks: High silica content, rich in quartz and feldspar (e.g., Granite, Rhyolite).

Source: Physical Geology, First University of Saskatchewan Edition

Source: Physical Geology, First University of Saskatchewan Edition

Shapes of Intrusive Bodies

• Plutons: Large, deep-seated igneous bodies.
• Dikes: Vertical sheets of igneous rock cutting through other rock layers.
• Sills: Horizontal sheets of igneous rock between layers of sedimentary rock.
• Batholiths: Very large, irregular intrusive bodies.
• Laccoliths: Dome-shaped intrusions that cause overlying layers to bulge.

More Examples

Source: https://www.alexstrekeisen.it/

Summary

Source: Physical Geology, First University of Saskatchewan Edition